benzene & aromatic compounds by: dr. shatha alaqeel
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Benzene & Aromatic Compounds
By:Dr. Shatha alaqeel
Introduction• Name= Benzene
• Chemical formula= C6H6
• Structure = Cyclic planner• Hybridization= SP2 • Bond angles= 120o
• Bond Length= 1.39 A
• 1) Benzene can be written as a six-membered ring with alternating single and doublebonds (Kekulé structure).
• 2) Benzene reacts with substitution reactions not with addition reactions
The Aromatic Character• When to say this compound is
aromatic?• 1) If it I cyclic• 2) If it has π and σ alternating bonds• 3) If it is planar
4) Reacts with substitution not addition reactions
5) Contain (4n+2) π electronsThis is called Huckel’s RuleWhere n= 0,1,2,3,…….When n=0 π=2 n=1 π=6 n= 2 π=10 n= 3 π=14
n = 0 (4x0)+2 = 2 π elect. not aromatic
n = 2 (4x2)+2 = 10 π elect. aromatic
n = 1 (4x1)+2 = 6 π electrons aromatic
N N O S....
..
......
Pyridine Pyrrole Furan Thiophene
ExcerciseApply Huckel,s rule to the following
N NH....
Naphthalene Quinoline 1H-Indole
Some Aromatic Ions
n = 0
+ -
not aromatic aromatic not aromatic
-n = 1
+
=
-
not aromatic aromatic
= +
+
n = 1
aromaticnot aromatic
etc...
Not aromatic (not planar)Not aromatic (does not
follow Huckel,s rule
Stability of BenzeneStability of Benzene
+ H2H = 28.6kcal\mol
+ 3 H2 H = 85.8 kcal\mol
+ 3 H2 H = 49.8 kcal\mol
(imaginary)
Benzene is more stable than cyclohexatriene by 36 kcal\mol : Resonance energy
Nomenclature of Organic Compounds
Cl Br I F
Chloro benzene Bromobenzene Iodobenzene Florobenzene
NO2
Ethylbenzene t-Butylbenzene Nitrobenzene
CH3
OHCHO
Tolune Phenol Benzaldehyde
CHCH2
COOH NH2
Styrene Benzoic acid Aniline
-C6H5, Ph- Φ
CH2
Benzyl group Phenyl group
OMe
Anisole 2-Phenylbutane PhenylcyclopentaneBiphenyl
Cl
Benzylchloride
Disubstituted Benzene
Br
Br
Br
Br
Br
Br
1,2-Dibromo-benzene 1,3-Dibromo-benzene1,4-Dibromo-benzene
ortho-Dibromo-benzene meta-Dibromo-benzene para-Dibromo-benzene
• 2) When the substituents are different, they are arranged alphabetically
Cl
F
I Br
NO2
1-Chloro-2-ethyl-benzene
O-Chloro ethylbenzene
1-Fluoro-4-iodo-benzene 1-Bromo-3-nitrobenzene
P-floro iodobenzene m-bromo nitrobenzene
If one of the substituents is part of a common named compound, then the disubstituted benzene is named as a derivative of the parent compound
OH
Cl
CH3
NO2
Br
COOH
2-Chlorophenolo-chlorophenol 4-nitrotolune
p-nitrotoluene
3-bromobenzoic acidm-bromobenzoic acid
Some disubstituted benzenes have common names
CH3
CH3
CH3
CH3
CH3
CH3o-xylene m-xylenep-xylene
Polysubstituted Benzene• Number so as to give the lowest possible numbering to
the substituents
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl Cl
1,2,3-trichlorobenzene
NOT 1,4,6-trichlorobenzene
1,2,4-trichlorobenzene 1,3,5-trichlorobenzene
The three trichlorobenzene are isomers
ClF
NO2OH
Br
Br
Br
2,4,6-Tribromophenol
CH3
NO2
NO2
O2N
2,4,6-trinitrotoluene
TNT
1-chloro-3-fluoro-5-nitrobenzene
Electrophilic Aromatic Substitution ReactionsElectrophilic Aromatic Substitution Reactions
COR
RCOCl, AlCl3Acylation
Summary of the electrophilic substitution reactions
Side-Chain Reactions of Aromatic CompoundsSide-Chain Reactions of Aromatic Compounds
a.a. Halogenation of an Alkyl Side Chain Halogenation of an Alkyl Side ChainCH3
Br2
UV light
Toluene
CH2Br
+ BrH
Benzyl Bromide
b.b. Oxidation of an Alkyl Side Chain Oxidation of an Alkyl Side ChainCH3
KMnO4
Toluene
COOH
Benzoic acid
CH2CH3KMnO4
COOH
Benzoic acid
+ + OH2CO2
Orientation effects of substituents in electrophilic Orientation effects of substituents in electrophilic aromatic substitution reactions of monosubstituted aromatic substitution reactions of monosubstituted
BenzenesBenzenes
Ortho , para directors Meta directors
-OH, -OR-NH2, -NHR, -NR2-C6H5-CH3, -R (alkyl)-F, -Cl, -Br, -I
-NO2-SO3H-COOH, -COOR-CHO, -COR-CN
OH
HNO3 / H2SO4
OH OH
+
NO2
NO2o-Nitrophenol 53 % p-Nitrophenol
47 %NO2
SO3 / H2SO4
OH
m-Nitrobezenesulfonic acid
SO3H
Orientation effects of substituents in Orientation effects of substituents in electrophilic aromatic substitution electrophilic aromatic substitution
reactions of monosubstituted Benzenesreactions of monosubstituted Benzenes Alkyl groups and groups with lone pairsAlkyl groups and groups with lone pairs direct new groups to ortho-, para- direct new groups to ortho-, para- positions and positions and speed-up speed-up the reaction the reaction (activating groups).(activating groups). HalogensHalogens direct new groups to ortho-, para- positions direct new groups to ortho-, para- positions but it it slow downslow down the the reaction reaction (deactivating).(deactivating). Electron withdrawing groupsElectron withdrawing groups such as nitro, nitrile, and carbonyl direct new such as nitro, nitrile, and carbonyl direct new groups to the meta-position and groups to the meta-position and slowslow the reaction the reaction down down (deactivating).(deactivating).
Thus the order of reactivity of benzene and monosubstituted benzene derivatives as in Thus the order of reactivity of benzene and monosubstituted benzene derivatives as in the followingthe following
Substituted benzene with Substituted benzene with o,po,p directors directors >> Benzene Benzene >> Halobenzene derivatives Halobenzene derivatives >> Substituted benzene with Substituted benzene with m-m- directors directors
OH
HNO3 / H2SO4
OH OH
+
NO2
NO2
COOH
HNO3 / H2SO4
COOH
NO2
COCH3
Br2 / FeBr3
COCH3
Br